The wastewater can be domestic, municipal, trade or industrial wastewater. The fluid can be, for example, water, an aqueous solution, alcohol or oil. Heat is transferred from the wastewater to the fluid to either cool the wastewater or heat the fluid. In particular, the heated fluid can be conducted in a circuit by a heat pump in order to render the wastewater heat utilizable for heating purposes. Inversely, however, heat can also be transferred from the fluid to the wastewater in order to heat the latter and thus make it easier to be treated.
The container can be a closed tank or an open basin. Wastewater can flow continuously through the container, or wastewater is supplied to and removed from the container in batches.
The heat exchanger is a hollow body through the interior of which the fluid flows and whose outer surface is in contact with the wastewater. Heat is transferred through at least one wall of the heat exchanger which wall preferably consists of sheeting that conducts heat well when the fluid and the wastewater have a different temperature. The thermal transfer capacity of the heat exchanger is proportional to the outer surface contacted by the wastewater, to the temperature difference of the two fluids and to a coefficient of thermal transfer (the k value). The k value is the inverse value of a thermal transfer resistance. This resistance is the stun of the resistances to the thermal transfer by the fluid on the wall, to the thermal conduction through the wall and to the thermal transfer from the wall into the surrounding wastewater. The resistance to the thermal conduction through the wall is proportional to the wall thickness and inversely proportional to the thermal conductivity of the wall material, thus, conditioned by the construction type of the heat exchanger. The resistances to the transfer of heat from the fluids into the wall and vice versa are, on the other hand, not only a function of the properties of the fluids (in particular of their thermal conductivities, viscosities and thermal capacities) but rather are also a function in particular of their flow behaviors. The inverse values of the inner and outer heat transfer resistances are coefficients of thermal transfer (alpha values).
Alpha values are low, that is, the heat transfer is poor, when fluids are at rest. The thermal transport is then dependent on the moderate thermal conduction as a consequence of diffusion and on natural convection, that is, a flow produced by the changed density of the fluids when they are heated or cooled off on the wall. Significantly better, that is, higher alpha values, can be achieved by forced convection, that is, artificially produced flows on the wall. Flows are characterized by their Reynolds number (Re), that is proportional to the flow rate and to a characteristic geometric length (e.g., the diameter of a tube) and inversely proportional to the kinematical viscosity of the fluid.
In the case of a low Re number, a flow is laminar and upon exceeding a critical Re number it changes into a turbulent flow, and the alpha value rises sharply. When heat exchangers are used, it should therefore be attempted to have both fluids flowing turbulently. It is simple to produce a turbulent flow in the fluid that is pumped through the heat exchanger in that the flow rate is selected to be sufficiently high. It is not very useful for the k value to further raise the inner alpha value more and more if the outer alpha value remains low. Then, the required thermal transfer capacity can only be achieved by a sufficiently large outer surface of the heat exchanger, which has the disadvantage that the heat exchanger becomes large and expensive. In order to avoid this, a convective flow, preferably a turbulent flow must be produced in the wastewater along the outer surface of the heat exchanger.
This flow should preferably pass over the outer surface, i.e., it should have a direction approximately parallel to the outer surface. The alpha value is raised by a defined and high flow rate over the outer surface of the heat exchanger and problematic solids and coatings are washed off the outer surface, for which reason the outer surface can be dimensioned to be small and the cost of the heat exchanger remains low.
It is state of the art to incorporate heat exchangers in wastewater conduits or drains in order to transfer heat between the wastewater and a fluid flowing through the heat exchanger. The wastewater flows in the conduit or drain past the outer surface of the heat exchanger. However, the flow rate is a function of the wastewater throughput through the conduit or the drain as well as of the level in it. Neither the flow rate nor the level are constant, so that no defined flow is present. The flow is normally laminar. Turbulent flow conditions prevail only when the flow rate has been greatly elevated, for example, after an occurrence of rain. As a result of the normally low alpha value on the surface of the heat exchanger, it must have a large surface. They are susceptible to contamination, especially if they are incorporated into raw wastewater conduits.
The published application DE 101 56 253 A1 teaches the integration of a heat exchanger into an industrial water tank in order to be able to make further use of the thermal energy present in the industrial water at another location. For this, the industrial water must be intermediately stored in the tank, during which it is preferably churned by blowing in air. As a result of this churning, the industrial water flows past the outer surface of the heat exchanger, which should improve the exchange of heat and at the same time wash contaminations off the outer surface of the heat exchanger.
Also, the published application DE 36 05 585 A1 teaches a heat exchanger incorporated in a wastewater container on the outer surface of which heat exchanger a convective flow is produced. In this instance the flow is produced by moving flexible walls of the wastewater container.
European patent EP 0 174 554 B1 also teaches a heat exchanger incorporated in a container for contaminated water in which a wash gas is blown into the water under the heat exchanger in order to clean the outer surface of the housing and to improve the thermal transfer.
According to these publications, convective flows are produced on the outer surfaces of heat exchangers for the purpose of improving the thermal transfer and cleaning contaminants off the outer surfaces. The heat exchangers here are always housed in a container arranged extra for this purpose in which the water is intermediately stored or must be conducted through a relatively expensive flow conduit which for its part is formed by a plurality of heat exchangers. However, these apparatuses significantly increase the investment and operating costs of the heat exchangers.
The present invention has the problem of making available a process and an apparatus for carrying out the process that do not have the cited disadvantages and makes possible a simple and economical utilization of the thermal energy present in wastewater. in particular, the invention has the problem of using already existing apparatuses additionally for the thermal exchange so that the energy balance can be further improved.
This problem is solved by the process and the apparatus in accordance with the features of the independent claims.